Re: Can light from different lasers interfere?

	This is a very interesting question you asked.  I had an idea about 
what the answer was, but after some digging, I found a lot more about it 
than I expected!  I guess we'll both learn something from this one.

	In short, yes light from different lasers can interfere - in fact, 
it is done quite often (I will explain this more later).  I do not have the 
"Feynman Lectures" that you quote from, so I could not read more on that 
topic.  However, what you read seems to be alluding to the Heisnburg 
Uncertainty Principle.  This essentially says that you can't possibly 
distinguish between the photons and still have the interference pattern 
exist.  The minute you make a measurement of the photon, the wavefuction 
will collapse........to put it more colloquially, you can't have your cake 
and eat it too!  	;-)	

	The reason you will not see this happen with classical waves is 
because this phenomenom is a quantum effect.  Quantum effects only come 
into play when you deal with atomic level actions.....the effect breaks 
down in the macroscopic world.

	Now, getting back to the lasers, two different lasers will never 
have the same output.  They have different power source and will have 
numerous other minute differences that will lead to slightly different 
linewidths.  (Linewidths are the range of output wavelengths that a laser 
produces.  Its profile looks similar to a Bell Curve - the center frequency 
will be the characteristic wavelength of the laser.  eg. a HeNe laser has a 
center frequency of 632.8nm)

	To quote a friend of mine (Kent Nickerson):  When "two equal 
powered monochromatic beams (as is produced with a beamsplitter from a 
single monochromatic source) combine (as done in an interferometer) they 
yield a static interference pattern. This is the same as the "standing 
wave" pattern observed when a radio frequency signal reflects back upon 
itself. If one of the sources (radio, optic, or any wave source) differs 
from the other in frequency by dF, the pattern will modulate at frequency 
dF."

	(dF equals the difference between the center wavelenths of the two 
lasers)

	This modulation is a result of the superposition of the two 
waveforms.  The low-frequency wave will serve as an envelope modulating the 
high-frequency wave.  (Pedrotti, 1993)  The two combined will produce a 
"beat" frequency that is equal to dF.  A good way to imagine this is to 
think of two tuning forks of slightly different pitches.  If you hear them 
individually, you will just hear the tone of that individual fork.  If you 
hear them together, you will hear the two tones, along with a sinusoidally 
cyclical combination of the two waves combined (sounding sort of like a 
"whomp, whomp, whomp......").

	Along with this superposition, the waves will also produce an 
interference pattern.  However, like my friend Kent said above, due to the 
superimposed waves, the pattern will modulate at a frequency of dF.

	Finally, also courtesy of my friend Kent, is a novel application 
for this principle:  One "application involves the testing of fast 
photodetectors for response to light modulated at, say, 50 GHz. This is 
very difficult - both in finding a laser that can be modulated that quickly 
and generating a 50 GHz electrical drive for the laser.
However, the optical frequencies of a laser radiating at 800nm and a 
similar one tuned to 800.1nm differ by 50 GHz. The 50 GHz modulated 
interference pattern made by shining both these beams onto the detector can 
be used to test the detector."

	I hope this explaination has helped you.  If you have any other 
questions please feel free to email me.


	Kurt Frost
	kfrost@sympatico.ca



REFERENCES:

Pedrotti, Frank and Leno Pedrotti.  1993.  Introduction to Optics.  New 
Jersey: Prentice Hall.

Dr. Dan Cassidy.  Deparment of Engineering Physics, McMaster University, 
Hamilton, Ontario, Canada.  Personal Conference.

Kent Nickerson, M.Eng (EE).  RF Researcher.  Waterloo, Ontario, Canada.  
Personal Conference.